Datensatz

Zusammenfassung

Water-rock interactions and other mineral reactions, such as dehydration processes, have a fundamental impact onstructural properties of a rock formation, such as porosity and pore connectivity, and are thus highly relevant fortransport and storage processes in crustal settings. Fluid-rock interactions at greater depths under higher pressureand temperature conditions are inaccessible to direct observation. However, the breakdown of minerals as wellas the formation of new ones changes the availability of charge carriers in the pore fluid, especially at elevatedtemperature. The availability of charge carriers and their mobility determine the electrical rock properties besidesthe pore structure. Studies on electrical properties of rock under controlled laboratory conditions may help toincrease our understanding of these processes. The purpose of this study is to examine under which circumstancesphysical properties of rocks and pore fluids can be used as monitoring tools for fluid-related processes in high-temperature environments. We have performed reactive flow experiments on water-rock systems of various fluid torock ratios at flow rates ranging from 0.02 – 0.00002 ml/min and pT conditions of unconventional high-enthalpygeothermal reservoirs (T > 350◦C, pfluid= 25 MPa). Hydraulic and electrical properties were determined on lowto medium porous rocks. Additionally, the electrical properties of highly reactive systems were measured, wherewater was circulated around a rock core. The measurements were supplemented by a number of additional tests,comprising microstructural investigations as well as the chemical analysis of fluid samples, which were taken atevery temperature step. At low temperature (< 200◦C), both physical and chemical data show only slight fluid-rock interactions, whereas above 200◦C, continuously increasing Si concentrations in the fluid samples indicate abeginning mineral dissolution. In porous samples with high initial fluid-rock contact area this process is detectableas decreasing electrical formation factor. At near-critical conditions Si dissolution is going to accelerate and alsoAl is more intensively mobilized. In highly permeable systems, the release of charge carriers to the formationfluid is accompanied by a steep increase in electrical fluid conductivity by factor 7 within seconds. This points toan extensive and spontaneous increase in rock solubility. However, at supercritical conditions conductivities didnot remain steady and the electrical properties of porous supercritical fluid-rock systems are characterized by afluctuation of conductivities over a wide range. This indicates a dynamic interplay of the competing processesof mineral dissolution and new mineral formation, which are also evident from complementary micro-structuralinvestigations as well as chemical analyses of the percolated fluids. From SEM analyses it is apparent that thealteration of the solid material is most effective where fresh fluid is continuously flowing around the solid, whilestagnant fluids in low permeable samples led to a much less pervasive alteration of the solid. In consequence,resistivity contrasts are too low, to be detectable. However, the release of additional water due to dehydrationreactions can cause strong changes in electrical resistivity, especially in stagnant low permeability systems.